High brightness optical device

ABSTRACT

There is provided an optical device, comprising a display source; a light-diffuser; an imaging optical module, and an output aperture from the optical device characterized in that the light diffuser is an angular, non-uniform diffuser of light for increasing a portion of light emerging from the display source that passes through the output aperture. A method for improving the brightness of an optical display is also provided.

FIELD OF THE INVENTION

The present invention relates to an optical display system and method,and particularly to a display system and method for see-through displayapplications.

The invention can be implemented to advantage in a large number ofpersonal imaging applications, such as, for example, head-mounteddisplays (HMDs) and head-up displays (HUDs), hand-held displays, as wellas binoculars, monoculars and bioculars. In addition, the samearrangement can be utilized for image projection systems such as frontprojectors for conference rooms and rear projection TV screens.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inHMDs wherein an optical module serves both as an imaging lens and acombiner, in which a two-dimensional display is imaged to infinity andreflected into the eye of an observer. The image to be displayed isobtained directly from either a spatial light modulator (SLM) such as acathode ray tube (CRT), a liquid crystal display (LCD), an organic lightemitting diode array (OLED), or a scanning source and similar devices,or indirectly, by means of a relay lens or an optical fiber bundle. Thedisplay comprises an array of elements (pixels) imaged to infinity by acollimating lens and transmitted into the eye of the viewer by means ofa reflecting or partially reflecting surface acting as a combiner fornon-see-through and see-through applications, respectively. Usually, oneof the most important issues to be addressed while designing an HMD, isthe brightness of the optical system. This issue is mostly important forsee-through applications, where it is desired that the brightness of thedisplay will be comparable to that of the external scene.

The strive for high brightness has led to several different complexoptical solutions, all of which, on the one hand, are still notsufficiently bright for many practical applications, and, on the otherhand, suffer major drawbacks in terms of fabrication procedures andoperational conditions.

DISCLOSURE OF THE INVENTION

The present invention facilitates the structure and fabrication of veryhigh brightness display sources for, amongst other applications, HMDs.The invention allows an efficient use of the available light of theillumination source, i.e., a relatively high brightness system togetherwith a relatively low power consumption can be achieved. The opticalsystem offered by the present invention is particularly advantageousbecause it is substantially brighter than state-of-the-artimplementations and yet it can be readily incorporated, even intooptical systems having specialized configurations.

The invention also enables the construction of improved HUDs, which havebecome popular and now play an important role, not only in most moderncombat aircraft, but also in civilian aircraft, in which HUD systemsbecame a key component for low-visibility landing operation.Furthermore, there have been numerous suggestions for the use of HUDs inautomotive applications for potentially assisting the driver in drivingand navigation tasks. State-of-the-art HUDs, however, suffer fromseveral significant drawbacks. All HUD's of the current structuresrequire a display source that must be offset the brightness of theexternal scene to ensure that the projected data will be readable evenwith very bright ambient light. As a result, the present HUD systemsusually require complicated high brightness display sources which arenecessarily bulky, large, and require considerable installation space,which makes it inconvenient for installation and, at times, even unsafeto use.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD structure of the present invention, the combineris illuminated with a compact display source having high brightness andsmall power consumption. Hence, the overall system is very compact andcan be readily installed in a variety of configurations for a wide rangeof applications.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless Internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the end-user's device. Themobility requirement restricts the physical size of the displays, andthe result is a direct-display with a poor image viewing quality. Thepresent invention enables a physically compact display with a large andbright virtual image. This is a key feature in mobile communications,and especially for mobile Internet access, solving one of the mainlimitations for its practical implementation. Thereby the presentinvention enables the viewing of the digital content of a full formatInternet page within a small, hand-held device, such as a cellularphone.

Furthermore this invention is also suitable for use in the constructionof the illumination for front and rear projection devices. Here thedesign replaces three complex dichroic polarizing beam splitters withaccurate alignment requirements.

For all possible applications, the present invention is particularlyadvantageous for utilizing substrate-mode configuration, that is, for aconfiguration comprising a light-transmitting substrate having at leasttwo major surfaces and edges: optical means for coupling the light fromthe imaging module into said substrate by total internal reflection, andat least one partially reflecting surface located in the substrate forcoupling the light onto the viewer's eye. The combination of the presentinvention with a substrate-mode configuration results in a compact andconvenient optical system having high brightness and low powerconsumption.

It is therefore a broad object of the present invention, to alleviatethe drawbacks of state-of-the-art virtual image display devices and toprovide an optical display system and method having improvedperformance.

In accordance with the present invention, there is provided an opticaldevice, comprising a display source; a light-diffuser; an imagingoptical module, and an output aperture from the optical devicecharacterized in that said light diffuser is an angularly, non-uniformdiffuser of light for increasing a portion of light emerging from thedisplay source that passes through said output aperture.

The invention further provides a method for improving the brightness ofan optical display, comprising providing an optical display systemincluding a display source and an imaging module having an outputaperture, and optically increasing the portion of light emerging fromthe display source that passes through said output aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a side view of a display system in accordance with the presentinvention;

FIG. 2 is a diagram illustrating the footprint of the light, coupledinto the system pupil, on the front surface of the collimating lens,according to the present invention;

FIG. 3 is a side view of a device according to the present invention,utilizing an LCD light source;

FIG. 4 is a schematic illustration of a light diffuser consisting of anarray of light source pixels and lenses, in accordance with the presentinvention;

FIG. 5 illustrates an enlarged view of the display source and the firstlens of the optical system according to one embodiment of the presentinvention;

FIG. 6 illustrates a display system in accordance with the presentinvention, wherein the imaging system is a telecentric lens, and

FIG. 7 illustrates a display system in accordance with the presentinvention, wherein the light waves are coupled into a substrate-modeelement.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates an optical display system, wherein light waves 2emerging from a display source 4 are diffused by a light diffuser 6,that can be an integral part of the display source, such that each lightwave which is emerging from a single point in the display source isdiverged into a finite solid angle. Usually, a Lambertian lightdiffusing mechanism is preferred, that is, a diffuser wherein thebrightness is constant regardless of the angle from which it is viewed.The light waves are then imaged by an imaging module 8 and illuminate anoutput aperture 10 of the optical system. For direct view opticalsystems, this output aperture can be defined as a head-motion-box or aneye-motion-box for a biocular or a monocular respectively, that is, thelocation where the viewer can see the entire image simultaneously.Alternatively, for see-through optical systems, where the image isprojected into the viewer's eye(s) through an optical combiner, theoutput aperture is defined as the active area of the external surface onthe combiner's plane.

One of the major issues to be addressed while designing a display systemis the image's brightness as seen by the viewer. This issue isutilizable for see-through applications, where it is desired that thebrightness of the display will be comparable to that of the externalscene, to allow acceptable contrast ratio and convenient observationthrough the combiner. For most of the optical systems it is not possibleto ensure that the insertion loss of the system is small. For example,there are systems where the transmittance of the external view shouldexceed η (where η<1) and any color change of the original external sceneis not allowed. Therefore the image brightness reduces through thecombiner by a factor of 1/(1−η). In principle, high-brightness displaysources can offset this difficulty, and indeed there are display sourcessuch as CRTs and virtual retinal displays (VRDs) that can yield veryhigh brightness. Nevertheless, this approach necessarily has a practicallimitation. Not only are high-brightness display sources very expensive,they also have high power consumption with associated high electricalcurrents. Moreover, the size, volume and manufacturing costs of the highbrightness devices are usually high. Furthermore, even suchhigh-brightest displays reach an inherent limit in terms of the maximalbrightness that can be achieved. As an example, the brightness of a VRDis limited by the maximal output power of the laser source, which isusually less than 100 mW for diode lasers. As for other display sources4, for example, transmission LCDs, which are presently the most abundantsource for small displays, the back-illumination light power is limitedto avoid undesired effects like flaring, which decrease the resolutionand contrast ratio of the display. Other approaches are thereforerequired to optimize the use of the available light from the source.

Consequently, the present invention offers another method to improve theoverall brightness of the system by controlling the display sourcebrightness without changing the input power. As shown in FIG. 1, tomaximize the achievable brightness, it is desirable that most of thelight that emerges from the display source couples into the outputaperture 10 of the system. While FIG. 1 illustrates an optical layout ofthe imaging system, FIG. 2 illustrates the foot-print of the light,which is coupled into the output aperture 10, on the front surface 12 ofthe lens 14. Typically, most display sources exhibit a near-Lambertiandistribution of the emitted light. That is, the light power isessentially distributed uniformly over the entire angular spectrum of2π, steradians. As can be seen in FIGS. 1 and 2, however, only a smallportion of the angular illumination distribution of the display sourceis actually coupled into the output aperture 10. From each point sourceon the surface of the display, only a small cone of light of ˜20-30°illuminates the footprint on the front surface 12 and is thereforecoupled into the output aperture 10. Consequently, a significantincrease in the brightness can be achieved if the light emerging fromthe display is concentrated inside this cone.

One method to achieve such directionality in the source illumination isto use an angular selective diffuser 6 for the LCD, that is, a diffuser6 with a non-uniform diffusion mechanism, namely, a diffuser wherein itsbrightness depends on the angle from which it is viewed. FIG. 3illustrates an example of a display system where the display source 4 isa transmission LCD. The light which emerges from the light source 16 andis collimated by lens 18, illuminates an LCD display source 20. Theimage from the LCD is collimated and reflected by an optical component22 onto the output aperture 24. Usually, a conventional diffuser 6scatters the light uniformly in all directions. An angularly selectivediffuser can spread the light in such a way that the light from eachpoint on the surface of the display source diverges into the requiredangular cone. In this case the power that the LCD display 20 illuminatesremains the same. For a 20-30° cone, the diverging angle of the lightfor each point is reduced by a factor of more than 50 as compared to the2π steradians of a Lambertian source, and therefore the brightness ofthe light increases by the same factor. Hence, a significant improvementin the brightness of the system can be achieved with a minimalstructural and manufacturing effort and without increasing the powerconsumption of the system. It is of particular advantage to exploitholographic diffusers that can control with a great precision thedivergence angle of the scattered light waves and can as well achievehigher optical efficiencies than conventional diffusers.

The spatial coherence of the illuminating light source 16 is usuallyquite limited, i.e., the light source is not a point source but has afinite size. A typical value for a conventional LED is in the order of 1mm. As a result, the output wave from the collimating lens 18 cannot bea pure plane wave but rather a continuity of plane waves with an angularspread of approximately 10°. Hence, if the required divergence of thespreading cones from the display surface has to be 30°, then therequired divergence angle of the selective diffuser 6 should be of theorder of 20°.

A different embodiment is shown in FIG. 4, which is utilizable not onlyfor LCDs, but also for other display sources. Here, use is made of anarray of micro-lenses 18 _(l) to 18 _(n) that is aligned with the pixelsof the display source 16 _(l) to 16 _(n). For each pixel a micro-lensnarrows the diverging beam that emerges from that pixel into the desiredangular cone. In fact, this solution is more efficient when thefill-factor of the pixels is a small number, or when the achievableresolution of the display source is actually greater than the requiredresolution. Seen is an array of n pixels of light sources 16 _(l).to 16_(n), wherein an array of n micro-lenses 18 _(l).to .18 _(n) that isaligned with the pixels light sources 16 _(l).to 16 _(n), narrows thediverging beams that emerge from the pixels. An improved version of thissolution is to design the emitting distribution function of the pixelsin the pixel-array to make each pixel diverge into the required angle.For example, in OLED displays, efforts are usually made to increase thedivergence angle of the single LEDs in order to allow viewing from awide angle. For the instant display application, however, it isadvantageous to keep this divergence angle small, in the order of20-30°, in order to optimize the brightness of the system.

FIG. 5 illustrates an enlarged view of the display source 4 and thefirst lens 14 of the optical system illustrated in FIG. 1. As can beseen, only a small cone of light of ˜30° indeed diverges from each pointin the display source to illuminate the exit pupil, however, thepropagation direction of each cone is actually different. While for thecentral point of the source the chief ray 26 of the beam is normal tothe display plane and cone itself is symmetrical around the normal, thechief ray 28 of the marginal point is inclined at an angle of ˜20° tothe normal. In this case, there are three methods to ensure that therays from the entire display source will illuminate the exit pupil.Thus, the present invention utilizes a non-uniform light diffuser 6,which can be embodied by either an angular selective diffuser whereinthe diffusion direction depends on the exact location on the diffusersurface, or a micro-lenses array having a period which is slightlydifferent than that of the display pixel array. The fabrication of sucharrays are difficult, and thus, an alternative method utilizing adiffuser 6 having a divergence angle that covers the rays spreading fromthe entire display source, is proposed. Although this method is simpleto implement it is clear that it reduces the brightness of the system.The required divergence in the example given above is approximately 70°,hence, the output brightness is decreased by a factor of ˜5.5.

A different approach to solve these difficulties is to utilize atelecentric lens as the imaging module, that is, to use a lens in whichthe aperture stop is located at the front focus, resulting in the chiefrays being parallel to the optical axis in the display space. Usually,the image from a telecentric lens remains in focus over the same depthof field as that of a conventional lens working at the same f-number.Telecentric lenses provide constant magnification at any objectdistance, making accurate dimensional measurements over a larger rangeof object distances than conventional lenses. This property is importantfor gauging three-dimensional objects, or objects whose distance fromthe lens is not known precisely.

FIG. 6 illustrates a modified display system as compared to the systemillustrated in FIG. 1. The main modification is that the surfaces of theimaging module 8, and especially surface 12, are now structured toprovide a telecentric system. As can be seen, both the chief ray 26 ofthe central wave, as well as the chief rays 28 originating at the edgesof the display are normal to the display plane. A uniform divergenceangle of ˜30° is sufficient to ensure that the waves of the entiredisplay will cover the exit pupil. This can be obtained by using auniform angular selective diffuser 6 or with a micro-lens array, so thatthe optimal brightness can be reached.

The degree of telecentricity is usually measured by the angle of thechief ray in the corner of the field. In machine vision, a standardcommercial lens may have chief ray angles of 20 degrees or more, whereintelecentric lenses have chief ray angles less than 0.5 degree and sometelecentric lenses even have chief ray angles of less than 0.1 degree.The manufacturing of a lens having a high degree of telecentricity iscomplex which leads to high manufacturing efforts. For the presentinvention, however, the telecentricity is not required for themeasurement of accuracy and the degree of telecentricity is notcritical. Therefore, a simple lens with a telecentricity of a fewdegrees is sufficient to achieve the required optimal brightness.

A further manner to achieve the required illumination on the entrancepupil of a projecting module, is applicable mainly for LCD based opticalsystems. As can be seen in FIG. 3, the light from the source 16 iscollimated by the lens 18 into a plane wave, or rather into a continuityof plane waves. Instead of collimating the light source, however, it ispossible to focus it to a different focal plane. As can be seen in FIG.5, the chief ray 28 of the marginal point is inclined at an angle of 1820° to the normal. Assuming that the display source has a lateraldimension of 10 mm, the chief rays from the pixels of the display allconverge to a virtual point, located approximately 15 mm from thedisplay source. Therefore, by utilizing the lens 18 to converge thelight 16 to this point, the required light divergence from the displaysource can be achieved. In this specific case, in order to achieve thedesired illumination, the lens 18 should have a wide aperture and boththe light source 16 and the lens 14 should have a very large numericalaperture. By changing the design of the lens 12, however, such that thechief ray of the marginal point would be inclined at an angle of ˜10°,instead of 20°, to the normal, it is possible to achieve the requiredlight divergence on the display source 20. Hence, in this case a simplerillumination device can be constructed with no need for a complicatedtelecentric lens. In practice, the structure of an optical systemaccording to the present invention can compromise between the structuraland manufacturing efforts and the specified brightness according to thespecific requirements of the optical system.

The present invention thus facilitates the structure and fabrication ofvery high brightness display sources for, amongst other applications,optical systems with see-through capabilities where the brightness ofthe projected image is critical, such as HMDs and HUDs, where the lightfrom the display source reaches a single eye and both eyes of anobserver, respectively. In addition, the present invention can be usedfor the illumination of front and rear projection devices where highbrightness is required to allow convenient observation. Since thephotonic efficiency of the system can be high, that is, almost all thephotons emerge from the display source reach the system output aperture,the invention allows relatively high brightness systems together withrelatively low power consumption. For the HMD and the HUD applications,the present invention is particularly advantageous, by utilizinglight-guided optics configurations.

FIG. 7 illustrates an optical system 30 according to the presentinvention, wherein the projection module is a light-guide opticalelement (LOE) comprising a light-transmitting substrate 32 having atleast two major parallel surfaces 34, an optical coupler 36 for couplingthe light from the imaging module 8 via a light admitting surface 40into the substrate 32 by total internal reflection, and at least onepartially reflecting surface 38 located in said substrate for couplingthe light onto the viewer's eye-motion box 42. The combination of thepresent invention with an LOE configuration yields a very compact andconvenient optical system along with very high brightness and low powerconsumption.

The above-described optical means 36 is an example of a method forcoupling input waves into the substrate. Input waves could, however,also be coupled into the substrate by other optical means, including,but not limited to, folding prisms, fiber optic bundles, diffractiongratings, and the like.

Also, in the embodiment of FIG. 7, input waves and image waves arelocated on the same side of the substrate, however, input and imagewaves could just as well be located on the opposite sides of thesubstrate. Other applications, in which input waves could be coupledinto the substrate through one of the substrate's lateral edges, arealso envisioned.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1. An optical device, comprising: at least one light source; a display source collecting light from the light source, illuminating the display source and emitting light waves; at least one converging lens having a first surface; a light-diffuser interposed between the display source and the first surface of the lens for non-uniformly diffusing the light; and an imaging optical module; the optical device having an output aperture; light waves emerging from a single point on the display source being diverged into predetermined finite solid angles lower than 30° and illuminating the first surface of the lens, such that the light waves from the entire display source cover the output aperture; wherein the optical nature and structure of the combination of the light source, converging lens, display source and light diffuser, dictate the solid angles, which result in a specific brightness at said output aperture.
 2. The optical device according to claim 1, wherein the brightness of the light diffuser depends on the angle of viewing.
 3. The optical device according to claim 1, wherein the display source is a liquid crystal display.
 4. The optical device according to claim 1, wherein the light diffuser is an angular selective diffuser.
 5. The optical device according to claim 4, wherein the angular selective diffuser is a holographic diffuser.
 6. The optical device according to claim 4, wherein light from the angular selective diffuser illuminates the liquid crystal device.
 7. The optical device according to claim 1, wherein the display source has a plane and the optical axis of the display source is normal to said plane.
 8. The optical device according to claim 1, wherein the light diffuser constitutes a part of the display source.
 9. The optical device according to claim 8, wherein light from the light source is converged by the at least one converging lens, and wherein the light diffuser is an angular selective diffuser.
 10. The optical device according to claim 1, wherein light from the light source is collimated by at least one converging lens to illuminate the display source.
 11. The optical device according to claim 1, wherein the display source has a display plane, and chief rays of waves originating from the display source normal to the display plane.
 12. The optical device according to claim 1, wherein the display source has a plane, and chief rays of waves originating from the display source converge onto a virtual point located beyond the plane.
 13. The optical device according to claim 1, wherein images from the display source are imaged by the imaging optical module onto the output aperture.
 14. The optical device according to claim 1, wherein images from the display source are collimated by the imaging optical module onto the output aperture.
 15. The optical device according to claim 1, wherein the light diffuser diffuses light emerging from the display source into a predetermined direction, to reach an eye-motion body of an observer.
 16. The optical device according to claim 1, wherein the light source is a point source.
 17. The optical device according to claim 1, wherein the light source has a finite size.
 18. The optical device according to claim 1, wherein the light source is a light-emitting diode.
 19. The optical device according to claim 1, wherein light from the light source is collimated by the converging lens into a continuity of plane waves.
 20. The optical device according to claim 1, wherein light from the light source is converged by the converging lens into a continuity of waves focused onto a predefined focal plane.
 21. An optical device, comprising: at least one light source; a display source emitting light waves; at least one converging lens collecting light from the light source and illuminating the display source, the converging lens sharing, the same optical axis as the display source; a light diffuser for non-uniformly diffusing the light; an imaging optical module, and an output aperture from the optical device; each light wave emerging from a single point on the display source being diverged into a predetermined finite solid angle such that the light waves from the entire display source cover the output aperture; wherein optical nature and structure of the combination of the light source, converging lens, display source and light diffuser, dictate the solid angles, resulting in a specific brightness at the output aperture.
 22. The optical device according to claim 21, wherein the brightness of the light diffuser depends on the angle of viewing.
 23. The optical device according to claim 21, wherein the display source is a liquid crystal display.
 24. The optical device according to claim 21, wherein the light diffuser is an angular selective diffuser.
 25. The optical device according to claim 24, wherein light from the angularly selective diffuser illuminates the liquid crystal device.
 26. The optical device according to claim 21, wherein the angular selective diffuser is a holographic diffuser.
 27. The optical device according to claim 21, wherein the display source has a plane, and the optical axis of the display source is normal to the plane.
 28. The optical device according to claim 21, wherein the light diffuser constitutes a part of the display source.
 29. The optical device according to claim 28, wherein light from the light source is converged by at least one converging lens, and wherein the light diffuser is an angular selective diffuser.
 30. The optical device according to claim 21, wherein light from the light source is collimated by the at least one converging lens to illuminate the display source.
 31. An optical device, comprising: at least one light source; a display source emitting light waves; at least one converging lens collecting light from the light source and illuminating the display source; a light diffuser for non-uniformly diffusing the light; an imaging optical module the converging lens, display source and imaging optical module forming an on-axis system; and an output aperture from the optical device; each light wave emerging from a single point on the display source being diverged into a predetermined finite solid angle such that the light waves from the entire display source cover the output aperture; wherein optical nature and structure of the combination of the light source, converging lens, display source and light diffuser, dictate the solid angles, resulting in a specific brightness at the output aperture.
 32. The optical device according to claim 31, wherein the brightness of the light diffuser depends on the angle of viewing.
 33. The optical device according to claim 31, wherein the display source is a liquid crystal display.
 34. The optical device according to claim 31, wherein the light diffuser is an angular selective diffuser.
 35. The optical device according to claim 34, wherein the angular selective diffuser is a holographic diffuser.
 36. The optical device according to claim 34, wherein light from the angularly selective diffuser illuminates the liquid crystal device.
 37. The optical device according to claim 31, wherein the display source has a plane, and the optical axis of the display source is normal to the plane.
 38. The optical device according to claim 31, wherein the light diffuser constitutes a part of the display source and is an angular selective diffuser.
 39. The optical device according to claim 38, wherein light from the light source is converged by at least one converging lens, to illuminate the angular selective diffuser.
 40. The optical device according to claim 31, wherein light from the light source is collimated by at least one converging lens to illuminate the display source.
 41. An optical device comprising: at least one light source; a display source emitting light waves; at least one converging lens collecting light from the light source and illuminating the display source, the display source having a plane and light from the light source being converged by at least one converging lens to a virtual point located beyond the plane; a light diffuser for non-uniformly diffusing the light; an imaging optical module, and an output aperture from the optical device; each light wave emerging from a single point on the display source being diverged into a predetermined finite solid angle such that light waves from the entire display source cover the output aperture; wherein optical nature and structure of the combination of the light source, converging lens, display source and light diffuser, dictate the solid angles, resulting in a specific brightness at the output aperture.
 42. The optical device according to claim 41, wherein the brightness of the light diffuser depends on the angle of viewing.
 43. The optical device according to claim 41, wherein the display source is a liquid crystal display.
 44. The optical device according to claim 41, wherein the light diffuser is an angular selective diffuser.
 45. The optical device according to claim 44, wherein the angular selective diffuser is a holographic diffuser.
 46. The optical device according to claim 44, wherein light from the angularly selective diffuser illuminates the liquid crystal device.
 47. The optical device according to claim 41, wherein the display source has a plane and the optical axis of the display source is normal to the plane.
 48. The optical device according to claim 41, wherein the light diffuser constitutes a part of the display source.
 49. The optical device according to claim 48, wherein light from the light source is converged by at least one converging lens, and wherein the light diffuser is an angular selective diffuser.
 50. The optical device according to claim 41, wherein light from the light source is collimated by at least one converging lens to illuminate the display source. 